One prior art approach for preventing adhesion and anchorage of pressure-sensitive adhesive materials to substrates of paper or plastics is to form a cured coating of silicone composition on the substrate surface to impart release properties. The methods of forming a cured silicone coating on the substrate surface include a method of forming a release coating via addition reaction, which is widely employed because of satisfactory cure and an ability to match with a variety of release requirements covering from low-rate peel to high-rate peel. Also the method of forming a release coating via condensation reaction, which is implemented earlier in practice, is still used at the present since it has the advantage that it is insensitive to addition reaction catalyst poisons.
The method of forming a release coating via reaction cure encompasses a solvent type wherein a silicone composition is dissolved in organic solvent, an emulsion type wherein a silicone composition is dispersed in water using an emulsifier, and a solventless type composed solely of a silicone composition. Since the solvent type has the drawback that it is sometimes harmful to the human body or environment, the safety aspect pushes a changeover from the solvent type to the solventless or emulsion type in progress. The emulsion type is expected to find expanding utility in the future because it is not only safe, but also advantageous from the application aspect in that it can be arbitrarily diluted with water or arbitrarily mixed with another aqueous material or emulsion material.
In general, the curable silicone compositions for release paper are desired to have varying release forces depending on their purpose. In the application where tight release is necessary, a composition having added thereto an alkenyl-containing MQ resin is widely used. Herein M units designate R′3SiO1/2 units, Q units designate SiO4/2 units, and R′ is a monovalent hydrocarbon group (the same applies hereinafter).
However, the tight release effect of the alkenyl-containing MQ resin is not so high, and sometimes, the release force does not reach the desired level even when the resin is added in large amounts. Since the alkenyl-containing MQ resin is expensive, the addition of large amounts is unfavorable in cost. There is a need for a composition which can achieve a tight release effect when added in minute amounts. It is also known that the alkenyl-containing MQ resin has the tendency that the release force decreases with the lapse of time as compared with that developed immediately after cure. There is a need for an additive ensuring that the release force remains unchanged with the lapse of time.
JP-B H05-53183 (Patent Document 1) discloses a release paper-forming silicone composition comprising an alkenyl-containing MQ resin in admixture with alkenyl-free MQ resin. The change with time of release force is reduced, but the tight release effect is insufficient.
JP 2750896 (Patent Document 2) discloses a release paper-forming solvent type silicone composition of addition reaction type comprising an alkenyl-containing resin. Low-temperature cure and release force with a little change with time are achieved, but the release force is not so high because tightening of release is not intended.
JP 2742835 (Patent Document 3) discloses an addition reaction type organopolysiloxane composition having added thereto an equilibration reaction product of a vinyl-containing organopolysiloxane and an organopolysiloxane of the formula: (R′3SiO1/2)a(R′2SiO)b(R′SiO3/2)c(SiO4/2)d wherein R′ is a monovalent hydrocarbon group, a, b, c and d indicative of molar fractions of respective siloxane units are a=0.1 to 0.6, b=0 to 0.45, c=0 to 0.3, and d=0.3 to 2.0. The reaction product is deemed to be an MQ resin having vinyl-terminated siloxane bonded thereto.
However, Patent Document 3 refers nowhere to the optimum range of structure. In Example, 40 parts by weight of dimethylpolysiloxane having a degree of polymerization of 8,000 and containing vinyl at both ends and 60 parts by weight of a 30 wt % toluene solution of polyorganosiloxane consisting of (R′3SiO1/2)a units (M units) and (SiO4/2) units (Q units) in a molar ratio of 0.8/1 are heat treated at 100° C. for 5 hours in the presence of potassium hydroxide as catalyst, and the product is used to formulate a high-release additive composition. Because of its extremely high degree of polymerization, the product must be diluted with an organic solvent before it can be used. The tight release effect is about 2 to 3.8 times when 10 parts by weight of the high-release additive is blended, and is still insufficient.
As the method of gaining tight release by adding a polyorganosiloxane resin having pressure-sensitive adhesiveness to an addition reaction type organopolysiloxane composition, the following technique is reported.
JP-B H06-086582 (Patent Document 4) relates to a pressure-sensitive adhesive silicone protective coating agent comprising a curable silicone rubber and a product obtained from partial dehydration condensation of a both end hydroxyl-containing organopolysiloxane and a MQ unit-containing silicone resin. This needs a solvent because the silicone rubber is used. Patent Document 4 relates to the pressure-sensitive adhesive technology and refers nowhere to the tight release effect of release control agent for release paper. Also no descriptions are found other than the blending ratio of the silicone rubber and MQ unit-containing silicone resin.
JP-A H10-110156 (Patent Document 5) relates to a silicone-based pressure-sensitive adhesive based on a mixture or partial condensate of vinyl raw rubber and MQ resin. This needs a solvent because the silicone rubber is used. Patent Document 5 refers nowhere to the tight release effect of release control agent for release paper.
JP-A 2010-37557 (Patent Document 6) discloses a release modifier comprising an organopolysiloxane resin-organopolysiloxane condensation product that is obtained by the condensation reaction of (a1) 100 parts by weight of MQ-type resin in which the molar ratio of M units to Q units is 0.6 to 1.0 and the content of hydroxyl or alkoxy group is 0.3 to 2.0% by weight with (a2) 20 to 150 parts by weight of chain-form diorganosiloxane containing a hydroxyl or alkoxy group and having an average degree of polymerization of 100 to 1,000. Patent Document 6 describes that the release force measured at a low peel rate of 0.3 m/min in Example is increased only by a factor of 1.4 to 2.2 over Comparative Example free of condensation product. No sufficient tight release effect is obtained. This is presumably because component (a1) has a hydroxyl or alkoxy content which is as low as 0.3 to 2.0% by weight and offers few reaction sites with component (a2) and the product does not possess a crosslinked structure obtained from full condensation.